There are many applications for vibration sensors and accelerometers, i.e., devices that measure physical displacement in at least one direction. These vibration sensors may be coupled to an object to determine the extent of vibration and otherwise monitor the condition of the object. At present, displacement sensors generally fall into two categories: 1) piezoelectric sensors; 2) microelectromechanical systems (MEMS) sensors; and 3) optical sensors, which are most commonly optical fibers.
Piezoelectric sensors utilize electrical signals generated by the compression of a voltage-generating crystal or ceramic (a piezeoelectric substance) to measure displacement. Piezoelectric sensors cannot be used to measure static forces which result in a fixed charge on the piezoelectric material. Further, short term accuracy of piezoelectric sensors can vary without significant signal conditioning and electronic processing of signals. Such techniques, however, subject the piezoelectric sensor to substantial electromagnetic and radio frequency interference. Piezoelectric sensors that measure vibration are also subject to long term drift in accuracy due to material degradation. These sensors are also limited in the applications to which they may be applied as piezoelectric systems can exhibit problems in electrical signal strength and dynamic range, drawbacks that are amplified in high-electrical-noise environments. The internal resistance of piezoelectric materials is also highly sensitive to environmental factors (e.g., temperature) resulting in noise that limits resolution. Further, piezoelectric sensors tend to have high errors and low sensitivity at low to moderate frequency of vibration. Piezoelectric sensors with higher accuracy levels are complex and expensive to produce.
MEMS sensors similarly have several drawbacks. MEMS sensors often have small sensing chips that are subject to Brownian thermal noise. The measured acceleration value of a MEMs sensor is a product of displacement times angular resonant frequency squared so that a high-frequency vibration with very small displacement or low frequency vibration with a large displacement will result in very high acceleration values. For mechanical transfer function and resonant frequency bandwidths in the range of 0.1 Hz to 10 kHz, given the very low mass of a MEMS sensor chip and small MEMS displacements, the signal to noise ratio can be quickly dominated by Brownian Noise, which limits resolution of the sensor.
Currently available optical sensors utilize indirect measurements of motion, resulting in additional complexity. Specifically, optical sensors rely on various methods of deformation of an optical path traversed by a light beam (typically a coherent light beam as generated by a laser). Optical sensors, such as fiber sensors, are similarly suboptimal in terms of measurements, as they lack sensitivity and require complex analysis of the wavelength or phase shift of the light. This analysis must be performed using complex, expensive, and fragile instrumentation such as a spectrometer or optical interrogator. In addition, optical fibers utilized as optical sensors have fixed optical cavities that are of limited use for measuring vibration as noise is introduced over high frequency bandwidths.
The present invention is directed to overcoming these and other deficiencies in the art.